The present invention relates to a radio receiver for frequency shift keyed (FSK) signals on an RF (radio frequency) carrier.
In a conventional superhet receiver the image response of the receiver is a first-order response and must be filtered out. A continual trade-off must be made between selectivity and loss due to the finite "Q" of the filter elements. In small size equipments this problem becomes extreme as the obtainable "Q" is lower. Furthermore, even in single channel applications the several sections of filtering need to be individually adjusted thus, increasing assembly costs.
In the particular case of receivers for wide-area pocket paging systems all these problems are present simultaneously. Small size leads to low "Q", but low losses are required as sensitivity must be high to overcome the poor antenna performance arising from the requirement for small volume. Both size and cost favor an integrated circuit approach, but this is difficult to realize with a superhet. Also very low power consumption is of great importance in this application. In general, there is a direct trade-off between circuit bandwidth and power consumption and so it is most economical of current to obtain as much gain, and to perform the signal processing, at as low a frequency as possible.
One solution to the problems outlined hereinabove is given in U.S. Pat. No. 4,193,034 which discloses first and second signal paths, each signal path including a mixer circuit followed by a low pass filter and a limiting amplifier stage, a local oscillator running at the carrier frequency, the local oscillator output being applied to one mixer circuit direct, means for applying the oscillator output to the other mixer circuit with a 90.degree. phase shift, and a D-type clocked flip-flop to the D input of which is applied the output of one of the limiting amplifier stage and the output of the other limiting amplifier stage is applied to the clock input of the flip-flop.